We have recently identified two new molecules of the B7 family, B7-H3 and B7-H4. Preliminary data from our laboratory indicate that B7-H3 engages an activation-induced receptor on T cells to provide a costimulatory signal for proliferation and differentiation. In contrast, engagement of a putative receptor for B7-H4 also in activated T cells leads to inhibition of T cell proliferation, cytokine secretion and cell cycle arrest. The overall goal of our study is to elucidate cellular and molecular mechanisms of B7-H3 and B7-H4 in immune regulation and to manipulate these pathways to facilitate treatment of cancers and graft-versus- host diseases (GVHD). The central hypothesis of this proposal is that manipulation of B7-H3 and B7-H4 pathways could positively enhance T cell immunity against cancers and negatively prevent GVHD. To test this hypothesis, we will first explore the effect mechanisms and therapeutic potentials of B7-H3 pathways. Soluble B7-H3 fusion protein and gene transfer will be used to enhance B7-H3 costimulation as therapeutic approaches for treatment of cancers in animal models. Furthermore, neutralizing monoclonal antibodies and gene targeting mice of B7-H3 will be employed to explore potential therapeutics for prevention of GVHD. In contrast, fusion protein and gene delivery of B7-H4 will be utilized to enhance B7-H4 signal to suppress allogeneic T cell responses as an approach to prevent GVHD; gene targeting and monoclonal antibodies will be used to block the suppressive effect of B7-H4 to enhance tumor immunity. In addition, we will use bioinformatics, expression cloning and mass spectrometry techniques to identify the novel receptor for B7- H3 and B7-H4. Finally, we will use our expertise in structural biology to develop structure-based engineering of these B7 molecules to enhance therapeutic efficacy. We anticipate that these studies will provide a foundation for the development of new approaches for the prevention and immunotherapy of cancers and GVHD.